The present invention relates to a compact nuclear magnetic resonance (NMR) probe. It is used in the precise measurement of magnetic fields and in particular the earth's magnetic field. The probe according to the invention is of a known type and is e.g. described in French patent applications FR-A-1 447 226 and FR-A-2 098 624.
The principle by which these probes operates will be briefly described. When a liquid sample, whose atomic nuclei have a magnetic moment and a kinetic moment of a non-zero nature, is exposed to a magnetic field the nuclear magnetic moments tend to be aligned parallel or antiparallel to the field. The energy difference between these two states defines a nuclear resonance energy or a nuclear resonance frequency, which is generally in the low frequency (lf) range of approximately 1000 Hertz.
However, with the standard fields and at ordinary temperatures, the overall nuclear polarization (positive or negative) of a sample remains low and difficult to detect.
The OVERHAUSER-ABRAGAM effect makes it possible to significantly increase this polarization. For this purpose a suitable paramagnetic substance is dissolved in a solvent, said substance being chosen so as to have a non-paired electron giving rise to an excited electron level with a hyperfine structure with four sublevels. Generally the pumping frequency is in the high frequency (hf) range of a few dozen megahertz.
The dipolar coupling between the electron spin of the thus excited paramagnetic substance and the nuclear spin of the solvent considerably increases the polarization of the latter. In accordance with the excited electron transition, the positive or negative nuclear polarization of the solvent is aided.
This procedure is further improved by using a double effect. A first radical solution (i.e. a solvent with a paramagnetic substance) is exposed to a high frequency, which saturates the electron level aiding the positive polarization of the solvent, whereas a second radical solution is exposed to a high frequency, which saturates the electron level aiding the negative polarization of the solvent.
In the first case, an excitation signal at the nuclear resonance frequency applied to the sample will be absorbed by the latter, whereas in the second case an excitation signal at said same frequency will lead to a stimulated emission at the resonance frequency. The sampling windings around the first and second solutions will then supply voltages of the same frequency, but of opposite phases. A connection in opposition will make it possible to form the sum thereof. All the interference signals induced in these windings and which have the same phase will be cancelled out.
Such a double effect probe can function with two different solutions and a single hf excitation frequency, provided that the absorption spectra of the two solutions are reciprocally displaced in such a way that the single frequency corresponds to the positive polarization in the one case and the negative polarization in the other.
However, a double effect probe can also function with the same solution subdivided into two samples and by applying to said two samples two different frequencies, in order to separately saturate the two sublevels of the paramagnetic substance.
Finally, by a final improvement, the signal supplied by the probe, which is at the nuclear resonance frequency, can be reinjected in the same way as the excitation signal of the samples into a loop connection, which then functions as an oscillator. In this way a probe of the spin coupling, double effect oscillator type is obtained. The probe according to the invention is of this type.